

The primary aim of the present book is to supply for students 

 beginning the study of Theoretical Mechanics a course of such a 

 nature as shall emphasize the fundamental physical principles of 

 the subject. Different students will of course approach the study 

 of mechanics with different interests, different aims, and different 

 amounts of mathematical equipment, so that it may not be possible 

 to produce a single book which shall exactly fit the requirements 

 of every class of student. But I believe that all students of me- 

 chanics, no matter what their aims and intentions may be, will 

 be in the same position in one respect, namely that they will best 

 begin the study of the subject by trying to acquire a firm grasp 

 of the physical principles, leaving aside at first all mathematical 

 developments and all practical applications, except in so far as these 

 contribute to the elucidation of the fundamental physical principles. 



I am aware that this belief is not held by all teachers of 

 mechanics, some of whom regard the laws of mechanics simply as 

 working rules to be acquired as rapidly as possible for their utili- 

 tarian value, while others appear to regard them in the same light 

 as the rules of a game, the game consisting in the solution of 

 mathematical puzzles, most of which have no conceivable refer- 

 ence to the facts of nature. I find it hard to believe that there 

 can be any considerable class of students for whom either of these 

 points of view is the best. As regards the former, I feel that a 

 student who cannot get, or does not wish to get, a clear under- 

 standing of mechanical principles would be well advised not to 

 enter a profession in which his work will consist in the handling 

 of mechanical problems ; and as regards the latter, that a student 

 who wishes merely to obtain material for puzzle solving would do 

 better to turn his attention to chess or double acrostics. 



iii 



